For a wavelength monitor, there is a scheme in which an etalon filter is used as the wavelength monitor that detects an emission wavelength of a semiconductor laser. The etalon filter includes two flat reflecting surfaces, and with the flat reflecting surfaces, the etalon filter resonate light between the reflecting surfaces to transmit or reflect a specific wavelength. A solid etalon filter, in particular, is possible to be structured with a single part because such as structure can be obtained using an optically transparent material that a reflecting film is formed on an input/output facet. In a following description, a wavelength monitor using the solid etalon filter is explained.
The etalon filter has a characteristic in which a transmissivity varies depending on a wavelength. Therefore, it is possible to obtain a wavelength discriminating signal that is intensity information converted from wavelength information by making a laser light pass through the etalon filter. In this case, it is an important condition for monitoring the wavelength with high precision that a characteristic for discriminating the wavelength does not vary as a condition of an external environment, such as ambient temperature, changes. However, temperature dependence of the characteristic for discriminating the wavelength is dependent on a characteristic of a material of the etalon filter, and is expressed as
                                          ⅆ            λ                                ⅆ            T                          =                                                                              ⅆ                  n                                /                                  ⅆ                  T                                            +                              α                ⁢                                                                  ⁢                n                                      n                    ·          λ                                    (        1        )            where dn/dT is temperature-dependent variation of a refractive index, and α is a linear expansion coefficient.
Materials generally used for the etalon includes glass.
However, with glass available at present, a numerator of on right side of the Eq. (1) is not zero. Therefore, in glass as a etalon material, there is a problem in which a wavelength discriminating characteristic varies according to a temperature change.
Many studies on development of an etalon filter that has no temperature dependence have been conducted up to now. In conventional technologies for realizing an etalon filter that has no temperature dependence, a material linear expansion coefficient compensates a ratio of change of the refractive index with respect to temperature (see, for example, U.S. Pat. No. 6,452,725). FIG. 1 is a schematic for illustrating an axis structure and a direction of an incident axis of the etalon filter described in U.S. Pat. No. 6,452,725. In an etalon filter 100, for example, a birefringent crystal LiSAF (LiSrAIF6) is used as the material of which a linear expansion coefficient compensates for a ratio of change of the refractive index with respect to the temperature. A reflecting mirror mechanism is adopted on a laser-light incident plane and an output plane of the etalon filter 100. Because the LiSAF crystal is a single-axis birefringent crystal, the LiSAF crystal has an optical anisotropy, and includes an optical axis (hereinafter, “c-axis”) and two axes (hereinafter, “a-axis” and “b-axis”) having a refractive index and a linear expansion coefficient different from those of the c-axis. Therefore, when a laser incident angle with respect to the c-axis is θ, an optical length change of the etalon filter 100 due to a temperature change is expressed as
                                                                                          ⅆ                  nL                                                                      ⅆ                    Δ                                    ⁢                                                                          ⁢                  T                                            =                                                                    ⅆ                                                                                                                                            ⅆ                      Δ                                        ⁢                                                                                  ⁢                    T                                                  ⁢                                                                                                    [                                                                              (                                                                                          n                                c                                                            +                                                                                                dn                                  c                                                                ⁢                                                                                                                                  ⁢                                Δ                                ⁢                                                                                                                                  ⁢                                T                                                                                      ⁢                                                                                                                  )                                                    ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                              θ                              )                                                                                                      ]                                            2                                        +                                                                  [                                                                              (                                                                                          n                                ab                                                            +                                                                                                dn                                  ab                                                                ⁢                                                                                                                                  ⁢                                Δ                                ⁢                                                                                                                                  ⁢                                T                                                                                      ⁢                                                                                                                  )                                                    ⁢                                                                                                          ⁢                                                      sin                            ⁡                                                          (                              θ                              )                                                                                                      ]                                            2                                                                                                                                                                                                    [                                                                  (                                                  1                          +                                                                                    dn                              c                                                        ⁢                                                                                                                  ⁢                            Δ                            ⁢                                                                                                                  ⁢                            T                                                                          )                                            ⁢                                                                                          ⁢                                              cos                        ⁡                                                  (                          θ                          )                                                                                      ]                                    2                                +                                                      [                                                                  (                                                  1                          +                                                                                    dn                              ab                                                        ⁢                            Δ                            ⁢                                                                                                                  ⁢                            T                                                                          )                                            ⁢                                                                                          ⁢                                              sin                        ⁡                                                  (                          θ                          )                                                                                      ]                                    2                                                                                        (        2        )            where nL is the optical length, nc is a refractive index experienced by a laser light having a polarizing plane that is parallel to a plane defined by the c-axis and a direction of the optical axis, nab is a refractive index experienced by a laser light having a polarizing plane that is the same as a plane perpendicular to nc, ΔT is the temperature change, dnc, is shift of the refractive index nc per unit temperature change, and dnab is shift of the refractive index nab per unit temperature change.
The optical length nL with which zero is obtained in Eq. (2) by changing the laser-light incident angle Δ with respect to the c-axis is the condition for achieving no temperature dependence, i.e., the condition for obtaining a wavelength discriminating characteristic that does not change with a temperature change. In this example of the conventional technology, setting θ=36.55° using LiSAF for the etalon material satisfies a condition with which the etalon filter 100 has no temperature dependence.
However, a fixing of the etalon filter 100 causes a change of the optical characteristic of the etalon filter 100 when the etalon filter 100 is installed in a wavelength monitor or in a semiconductor laser module including the wavelength monitor. As a result, parameters of the etalon filter 100 are deviated from the condition for achieving no temperature dependence, and when the etalon filter 100 is used for the wavelength monitor, the wavelength discriminating characteristic changes with a temperature change.
Furthermore, a change of the temperature characteristic by the laser-light incident angle θ with respect to the c-axis greatly shifts at a vicinity of the angle θ with which the condition for achieving no temperature dependence is satisfied, the condition for achieving no temperature dependence is easily deviated by a subtle change of an angle of an incident laser light.
In view of the above, and it is an object of the present invention to provide a wavelength filter that satisfies the condition for achieving no temperature dependence even when the wavelength filter is fixed at a holder and a wavelength monitor using the wavelength filter.